[Constraint solver] Do not allow unavailable decls to be favored.

lib/Sema/CSSimplify.cpp

@@ -2980,8 +2980,8 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
           }
         }
       }
-      
-      // If the invocation's argument expression has a favored constraint,
+
+      // If the invocation's argument expression has a favored type,
       // use that information to determine whether a specific overload for
       // the initializer should be favored.
       if (favoredType && result.FavoredChoice == ~0U) {
@@ -2996,7 +2996,8 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
             argType = ctor.Decl->getInnermostDeclContext()
                 ->mapTypeIntoContext(argType);
             if (argType->isEqual(favoredType))
-              result.FavoredChoice = result.ViableCandidates.size();
+              if (!ctor->getAttrs().isUnavailable(getASTContext()))
+                result.FavoredChoice = result.ViableCandidates.size();
           }
         }
       }

lib/Sema/CSSolver.cpp

@@ -2338,11 +2338,21 @@ bool ConstraintSystem::solveRec(SmallVectorImpl<Solution> &solutions,
 /// Whether we should short-circuit a disjunction that already has a
 /// solution when we encounter the given constraint.
 static bool shortCircuitDisjunctionAt(Constraint *constraint,
-                                      Constraint *successfulConstraint) {
-
+                                      Constraint *successfulConstraint,
+                                      ASTContext &ctx) {
+
   // If the successfully applied constraint is favored, we'll consider that to
   // be the "best".
   if (successfulConstraint->isFavored() && !constraint->isFavored()) {
+#if !defined(NDEBUG)
+    if (successfulConstraint->getKind() == ConstraintKind::BindOverload) {
+      auto overloadChoice = successfulConstraint->getOverloadChoice();
+      assert((!overloadChoice.isDecl() ||
+              !overloadChoice.getDecl()->getAttrs().isUnavailable(ctx)) &&
+             "Unavailable decl should not be favored!");
+    }
+#endif
+
     return true;
   }
 
@@ -2533,7 +2543,8 @@ bool ConstraintSystem::solveSimplified(
     // short-circuit the disjunction.
     if (lastSolvedChoice) {
       auto *lastChoice = lastSolvedChoice->getConstraint();
-      if (shortCircuitDisjunctionAt(&currentChoice, lastChoice))
+      if (shortCircuitDisjunctionAt(&currentChoice, lastChoice,
+                                    getASTContext()))
         break;
     }
 

lib/Sema/ConstraintSystem.cpp

@@ -1314,7 +1314,11 @@ void ConstraintSystem::addOverloadSet(Type boundType,
                                        *favoredChoice,
                                        useDC,
                                        locator);
-
+
+    assert((!favoredChoice->isDecl() ||
+            !favoredChoice->getDecl()->getAttrs().isUnavailable(
+                getASTContext())) &&
+           "Cannot make unavailable decl favored!");
     bindOverloadConstraint->setFavored();
 
     overloads.push_back(bindOverloadConstraint);

test/Constraints/availability.swift

@@ -0,0 +1,17 @@
+// RUN: %target-typecheck-verify-swift -swift-version 4
+
+// Ensure that we do not select the unavailable failable init as the
+// only solution and then fail to typecheck.
+protocol P {}
+
+class C : P {
+  @available(swift, obsoleted: 4)
+  public init?(_ c: C) {
+  }
+
+  public init<T : P>(_ c: T) {}
+}
+
+func f(c: C) {
+  let _: C? = C(c)
+}